1. Field of the Invention
The invention concerns a method and a device for securing an area monitoring air pressure.
2. Description of the Technology
DE-OS No. 27 14 942 and No. 27 29 710 and EP-OS No. 39 142 show alarm systems where an overpressure or a reduced pressure is generated and maintained in the monitored space or door cavity. Upon opening or a violation of a door or a window, pressure drops or rises of the space or cavity was overpressured or under a reduced pressure respectively. This change in pressure is used to actuate an alarm. Alarm systems of this type are expensive particularly in view of the energy required to maintain the overpressure or reduced pressure. In addition, the system is inactive in case of a power failure. A system of this type is susceptible to external effects, such as gusts of wind or strong external air movements, so that malfunctions may occur.
The electric alarm circuit shown in DE-OS No. 19 16 472, detects air movements generated by the movements of an intruder by a bolometer amplifier circuit and utilizes this air movement to actuate an alarm. The alarm is not actuated by measurement of air pressures or pressure differences, but merely by air movements which may be caused by the opening or violation of doors or windows and by strong wind pressures or air circulations generated by thermal effects. The actuation of the alarm is therefore not defined sufficiently or specifically enough.
In the space alarm system according to DE-OS No. 22 37 613, a sound field is generated in the room to be monitored. The sound field may be set up by a speaker, preferably in a frequency range below the audible limit of 15 Hz. The sound field measured by a microphone type pressure transducer. The alarm is actuated by phase, frequency or amplitude changes caused by intruders moving about in the room or the opening or closing of doors or windows. This system may be mislead by slow opening or closing of doors or windows. It is also susceptible to false alarms caused by changes due to external effects, such as strong gusts of wind particularly in drafty rooms and by movements of air.
A burglar alarm system is shown in DE-OS No. 34 12 914 where the air pressure is measured inside the secured room. Low frequency changes, in particular in the range of 0.01 to 1 Hz, are filtered from the frequency spectrum. The alarm is actuated upon detection of such low frequency changes. While the practical testing of this system showed that, fundamentally, it has good sensitivity, its susceptibility to interference by environmental and external effects, such as thin walls, other air movements and, for example, air pressure changes in this frequency caused by passing trucks or aircraft flying overhead was large enough to render the safe actuating of the alarm free of interference impossible. Furthermore, the system is easily defeated by slow opening and closing of windows and doors.